1. Field of the Invention
The present invention relates to a direct-positive photographic image-forming internal latent image-type silver halide photographic light-sensitive material, and more particularly to an internal latent image-type layered-grain-structure silver halide photographic light-sensitive material having a high speed, high maximum density and low minimum density, and excellent storability and preservability for long periods of time.
2. Description of the Prior Art
There have heretofore been known various positive-type silver halide light-sensitive materials. Of these, non-surface-fogged, internal latent image-type silver halide light-sensitive materials, generally, are highly sensitive as compared to those light-sensitive materials of the prefogged silver halide emulsion type producing a positive image by destroying the fog nucleus (latent image) of an exposed area, utilizing the solarization or Herschel effect. Emulsions containing non-surface fog grains therefore advantageously usable in the direct positive-type light-sensitive material that needs to be highly sensitive.
The internal latent image-type direct-positive light-sensitive material, after being exposed imagewise, is subjected to surface development after or with fogging treatment, whereby a direct positive image can be obtained.
The mechanism of the formation of this direct-positive image, although still not clarified in detail, may be understood to a certain extent by making reference to, e.g., Mees and James, "The Theory of the Photographic Process," 3rd ed., p. 161 ("Desensitization by Internal Latent Images"), and the like.
There are many types of silver halide emulsions known as the internal latent image-type silver halide emulsion, which include those conversion emulsions as described in U.S. Pat. No. 2,592,250; those metallic ion doping emulsions or internal-chemically-sensitized layered-grain-structure-type emulsions as described in U.S. Pat. No. 3,206,313; Japanese Patent Examined Publication Nos. 29405/1968, 29012/1976; U.S. Pat. Nos. 3,317,322, 3,447,927, 3,531,291, 3,703,584, 3,761,266, 3,761,267 and 3,761,276; those emulsions in which silver chloride and silver bromide are deposited in the layered-grain-structure form as described in Japanese Patent Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 8524/1975 and 38525/1975; and those layered-grain-structure-type emulsions which use conversion emulsions as the core thereof as described in Japanese Patent O.P.I. Publication No. 127549/1980.
In these silver halide emulsions, generally, the core of the silver halide internally sensitized by chemical sensitization, metallic ion doping, conversion, or the like, is coated with a shell, and the surface of the silver halide particle, if necessary, is slightly sensitized.
However, it has now been found that the thus obtained emulsion leaves some problems to be solved with respect to the stability over the course of time.
Upon this, the stability in the course of time can be increased by, for example, increasing the thickness of the shell. On the other hand, however, the increase in the thickness is disadvantageous in respect that it results in the decrease in the maximum density or it deteriorates the developability. Chemically sensitized nuclei produced by chemically sensitizing the surface of silver halide particles, although capable of improving the problems of the developability and maximum density, tends to adversely affect the problems of the negative's speed and minimum density, so that the nucleus needs to be produced under correctly controlled conditions. And it is poor in the stability for storage for long periods; during which it increases the minimum density or deteriorates the maximum density of the resulting positive image. Therefore, in order to make practical use of emulsions of this kind in various photographic fields, the improvement on the photographic characteristics; i.e., the improvement on the sensitivity, decrease in the minimum density (Dmin), increase in the maximum density (Dmax), and besides, the improvement on the storability or preservability in the course of time are required.
Many attempts have been made to improve the storability with time of the internal latent image-type emulsion. For example, Japanese Patent O.P.I. Publication Nos. 138820/1975 and 21067/1980 disclose the use of mercapto-type compounds; No. 54437/1981 discloses the use of indazole compounds; No. 138631/1982 describes the use of polyvinylpyrolidones; and No. 66727/1978 describes the use of oxidizing agent compounds such as red prussiates. Further, Japanese Patent O.P.I. Publication No. 136641/1982 discloses the use of polyvinylpyrolidones at the time of the chemical ripening of the silver halide particle surface.
These compounds, however, should be used in a large quantity in order to exhibit their effect, and, if used that way, cause remarkable desensitization and deterioration of the density due to their development inhibiting effect. These compounds, therefore, are not considered suitable for practical use.